The present invention relates to a liquid crystal display device, and, more particularly, to a liquid crystal display device in which the contrast of an image is enhanced by preventing the reflection of external light.
A liquid crystal display device is used in a television receiver set, a personal computer, and as the display device for a portable terminal or the like. Since the liquid crystal display device is light in weight and exhibits a small power consumption, the liquid crystal display device is commonly used as a display device in a small electronic terminal, such as a mobile telephone or the like.
Further, since such a portable terminal is used outside of houses or buildings, a partial-transmissive-type liquid crystal display device is used. The partial-transmissive-type liquid crystal display device displays images by making use of external light when the outside environment is bright and displays images by making use of light from a backlight when the outside environment is dark (see Japanese Unexamined Patent Publication 350158/2001, for example). Further, even with respect to a liquid crystal display device which employs a full-transmissive-type panel, there is a liquid crystal display device which can produce a transmissive display which mainly uses light from a backlight and a reflective display which reflects light incident from an image observing side on a reflector of the backlight (see Japanese Unexamined Patent Publication 98960/2002 (paragraphs 0034 to 0043, FIG. 2 and FIG. 3) and Japanese Unexamined Patent Publication 98963/2002 (paragraphs 0001 to 0007, 0016 to 0017, FIG. 1, FIG. 3, FIG. 5, for example). Further, in many liquid crystal display devices, thin film transistors are used as switching elements.
Recently, to cope with the demand for a liquid crystal display device having a higher definition, the number of pixels of the liquid crystal display device has been increased. As a result of the increase in the number of pixels, thin film transistors which exhibit a high operational speed are required. In a liquid crystal display device having high definition polysilicon (polycrystalline silicon) is used in place of amorphous silicon as a semiconductor layer of the thin film transistor. With the use of polysilicon as the material of the semiconductor layer, the operational speed of the thin film transistor is increased, whereby images of high definition can be displayed.
Further, there is a known technique in which an upper-side background layer and a lower-side background layer are stacked on a glass substrate, laser beams are radiated to semiconductor thin films formed on the upper-side background layer, and the semiconductor thin films are crystallized (see Japanese Unexamined Patent Publication 132306/1994 (paragraphs 0002 to 0007, FIG. 1 to FIG. 4).
Usually, the thin film transistors are formed on the glass substrate and so-called alkali-free glass is used as a material of the glass substrate. This glass substrate includes impurities and the impurities intrude into the polysilicon film, so that the transistor characteristics of the thin film transistor formed on the substrate are degraded.
To suppress the intrusion of the impurities into the polysilicon film from the glass substrate, a background film made of silicon nitride, silicon oxide or the like is interposed between the glass substrate and the polysilicon film. The background film is formed over the whole surface of a panel and light-transmissive pixel electrodes are formed over the background film in addition to the thin film transistors. However, when the background film and the pixel electrodes are stacked, this gives rise to reflection of external light attributed to the difference in the refractive indices of the respective films.
In the conventional liquid crystal display device using the backlight, a background film is formed also in the light transmissive region. Accordingly, there has been a drawback in that, when the external light is reflected in the region where the light transmissive pixel electrodes are formed, the contrast of the images is lowered.
Further, in the partial-transmissive-type liquid crystal display device, the light reflective region and the light transmissive region are both formed in one pixel. Accordingly, when the image is displayed by making use of light from the backlight, the transmitting light is blocked by the reflective region, and, hence, the luminance of the screen is low. By adopting the full-transmissive panel structure having no reflective electrode in the pixel electrode and by reflecting the external light by use of the backlight, the luminance when the backlight is used can be enhanced. However, since such a display device is constituted by stacking the background film, the electrodes, the interlayer insulation films and the like, interface reflections attributed to the difference in refractive index occur on the interfaces of the respective films. Accordingly, with respect to the full-transmissive-type liquid crystal display device which reflects the external light by use of the backlight, there has been a drawback in that, when the images are displayed by the reflected light, an inverted image whose tint is inverted is displayed.